1. Field of the Invention
The present invention relates generally to an injection molding machine, and more particularly to an apparatus for applying vibrational repetitive movements to a melt of molding materials held inside the heating cylinder of an injection molding machine so as to improve the moldability of the melt.
2. Description of the Related Art
One of the present inventors has proposed an improved screw injection molding system in which low frequency vibrations are applied to a melt of molding materials to lower the viscosity of the melt, as disclosed in Japanese Patent Laid-open Publication No. HEI-11-170318. The disclosed screw injection molding system includes a needle valve mechanism which includes, as shown here in FIG. 8, a guide 32 engaged in a block 19 connecting a heating cylinder 11 and a discharge nozzle 18, a primary cylinder 34 connected to the guide 32 via a bracket 33, a primary piston 35 slidably disposed interiorly of the primary cylinder 34, a secondary cylinder 37 formed integrally with the primary piston 35, a secondary piston 38 slidably disposed interiorly of the secondary cylinder 37, front and rear piston rods 41 and 41 extending from the secondary piston 38 in a forward-and-rearward direction, an elongated rod 44 received in the front and rear piston rods 41, 42 and fixed to the latter via bushes 43, 43, a needle 31 formed at a distal end of the elongated rod 44, and an amplitude adjusting nut 46 mounted to a proximal end of the elongated rod 44 and locked in position by a lock nut 47. In FIG. 8 numeral 12 denotes a screw slidably and rotatably received in the heating cylinder 11.
In FIG. 8, the needle 31 of the needle valve mechanism 30 is placed in an open position in which the nozzle 18 of the heating cylinder 11 is opened. When the primary piston 35 is advanced or moved in the direction of the arrow a relative to the primary cylinder 34, the needle 31 moves to a closing position in which the nozzle 18 is closed. By thus moving the primary piston 35 back and forth relative to the primary cylinder 34, the nozzle 18 can be opened and closed by the needle 31.
While the needle 31 is held in the open position of FIG. 8, the secondary piston 38 is moved reciprocally (back and forth) in the direction of the arrow b relative to the secondary cylinder 37. In this instance, the needle 31 never closes a flow path of a molten material in its fully advanced position. In its half-open position, the needle 31 thus vibrates the molten material, thereby lowering the viscosity of the molten material. The amplitude of vibration of the needle 31 is determined by the distance h between the secondary cylinder 37 and the amplitude adjusting nut 46, which can be varied by turning the amplitude adjusting nut 46 rightwardly or leftwardly.
By thus moving the needle 31 reciprocally in the direction of the arrow b, low frequency vibrations can be applied from the tip of the needle 31 to the melt of molding materials. However, since the application of vibrations to the melt in the prior arrangement relies on minute reciprocal movement of the secondary piston 38 controlled hydraulically, a controller requires a servo valve installed in a hydraulic circuit. The servo valve is expensive per se and requires a complicated control circuit. This may render the controller complicated in construction and highly expensive to manufacture.
In addition, the frequency of vibrations must be changed according to the sort of molten materials used. This requirement may enhance the difficulties in achieving servo control of the secondary piston.